The present invention relates to a color image recording apparatus for recording a color image on a recording medium with the use of intermediate mask members and a photosensitive recording medium.
Various kinds of image recording apparatuses, such as color printer, have been known in the art. One of such apparatuses uses intermediate mask members. In the color recording apparatus using the mask members, light bearing an original image is subjected to color resolution by means of three color resolution filters of red (R), green (G) and blue (B) to split the light into three light beams having wavelengths of red, green and blue. Based on the three light beams, three intermediate mask members are produced by a laser printer, on which light shielding intermediate images are formed. Each of the mask members is superposed on a photosensititive recording medium and the latter is exposed to light through the mask member so that a latent image corresponding to the original image is formed on the photosensitive recording medium.
While such type of recording apparatus is advantageous in comparison with a recording apparatus of the type in which a photosensitive recording medium is directly exposed to a laser beam in that a modulation circuit for modulating a light signal is not required and that an optical system is simple in arrangement, because the recording medium is exposed to light by way of scanning the intermediate images.
In producing the mask members using the laser printer, dither method, such as Bayer method, has been used to enhance gradation reproducibility of the orginal image. In this method, the number of dots to be printed within one pixel on the mask member is determined in accordance with pixel density data in the original document or data regarding ratio of the amount of light reflected from the original document to the amount of light irradiated thereonto, whereupon a predetermined pattern is printed in the corresponding pixel.
However, due to the thickening of the dot caused by an edge effect inherent to an electrophotographic process or by a phenomenon occuring in the course of transferring and fixing processes, a desired image density cannot be obtained by the dither method when an image is produced by the laser printer. Specifically, the density of a half-tone image in the original document changes due to the edge effect or the dot thickening phenomenon. For example, with the laser printer having a resolution of 300 dots/inch, the size of one dot latent formed on a photosensitive drum by the irradiation of a laser beam thereonto is about 85 .mu.m in diameter. However, the size of the dot outputted from the laser printer is in the range of about 150 to 200 .mu.m in diameter. Further, in an area on the copied paper where black and white portions co-exists, a large amount of toner powders tends to be adhered to the boundary region in the side of the black portion. As a result, the amouunt of toner to be adhered to the inside of the black portion decreases and hence the density of the black portion is lowered.
As indicated by a dotted line a in FIG. 2, the relation between the number of dots contained within one pixel and the reflection ratio (or the transmission ratio) must theoretically be linear. In actuality, however, the relation between the number of the dots within one pixel on the image reproduced by the laser printer and the reflection ratio (or the transmisson ratio) is as shown by line b in FIG. 2 due to the reasons stated above.
Furthermore, when the photosensitive recording medium is exposed to light through the mask member, the relation between the reflection ratio (or transmission ratio) of the image on the mask member and the reflection ratio (or transmission ratio) of the image formed on a developer sheet is not linear as shown in FIG. 3 but is non-linear or distorted due to the fact that the light irradiated onto the mask member partially enters into the gap between the mask member and the photosensitive medium although the former is bought in facial contact with the latter. For the reasons stated above, gradation cannot be expressed sufficiently accurately.